Enhanced riser-based gas-lift apparatus

ABSTRACT

An apparatus may include a production riser (1), an eductor (2), a motive fluid line (3), a suction line (4), and a mixed fluid line (5). The production riser may extend into a wellbore and be configured to convey production fluid from the wellbore to a wellhead (7). The eductor (2) may be configured to mix a slip stream (9) of the production fluid (6) with high-pressure motive fluid (12), providing a mixed fluid (9). The motive fluid line may extend into the wellbore from a compressor to the eductor and be configured to provide the high-pressure motive fluid to the eductor. The suction line may be configured to provide the slip stream from the production riser to the eductor. The mixed fluid line may be configured to convey the mixed fluid from the eductor to the production riser. A method and system may employ such an apparatus.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.62/270,761, filed Dec. 22, 2015, which is incorporated herein byreference.

BACKGROUND

The present disclosure relates to an improved method and apparatus forhydrocarbon recovery from a well beneath a body of water. Moreparticularly, the present disclosure relates to a method and apparatusfor maintaining a controlled hydrostatic pressure in a subsea riser.

In recent years the search for oil and natural gas has extended intodeep waters overlying the continental shelves. In deep waters it iscommon practice to conduct drilling operations from floating vessels orfrom tall bottom-supported platforms. The floating vessel or platform isstationed over a well site and is equipped with a drill rig andassociated equipment. To conduct drilling operations from a floatingvessel or platform a large diameter riser pipe is employed which extendsfrom the surface down to a subsea wellhead on the ocean floor. The drillstring extends through the riser into blowout preventers positioned atopthe wellhead. The riser pipe serves to guide the drill string and toprovide a return conduit for circulating drilling fluids as well asproduction fluids once the drilling operation is complete.

In order to control pressure and/or enhance production, gas may beinjected into the riser in a technique called “gas lift.” A conventionalgas lift technique is described in U.S. Pat. No. 4,099,583 to Maus. Inthis technique, gas is introduced into the riser to reduce the averagefluid gradient. However, it is believed that the lightened fluid of thistechnique includes poorly mixed gas and liquid, allowing the gas portionto rise quickly without providing as much lift to the fluid as might bepossible using improved apparatus and methods.

U.S. Pat. No. 7,510,012 to Fernandes et al. proposes one solution toaddress the problem of bubbles that are deemed too large. In thisapproach, a bubble breaker assembly is used to decrease bubble size andprovide better distribution. However, such assemblies are intrusive anddo not allow passage of a pig through the main bore.

Other gas lift systems have been proposed to reduce hydrostatic head andincrease flow velocity to prevent liquid build-up. For example, U.S.Pat. Pub. No. 2015/0000926 to Beg et al. describes the use of a jet pumpdownstream of the wellhead to reduce wellhead pressure. However, suchjet pump is designed to lower the pressure at the surface which in factcan be additive to the reduction in pressure.

SUMMARY

An apparatus may include a production riser, an eductor, a motive fluidline, a suction line, and a mixed fluid line. The production riser mayextend into a wellbore and be configured to convey production fluid fromthe wellbore to a wellhead. The eductor may be configured to mix a slipstream of the production fluid with high-pressure motive fluid,providing a mixed fluid. The motive fluid line may extend into thewellbore from a compressor to the eductor and be configured to providethe high-pressure motive fluid to the eductor. The suction line may beconfigured to provide the slip stream from the production riser to theeductor. The mixed fluid line may be configured to convey the mixedfluid from the eductor to the production riser.

A method for reducing base pressure in a subsea riser may includeproviding an eductor upstream of a wellhead. The wellhead may beupstream of the subsea riser. The method may also include supplying theeductor with high-pressure motive fluid and supplying the eductor withproduction fluid. The method may involve, with the eductor, mixing thehigh-pressure motive fluid and the production fluid to produce a mixedfluid and introducing the mixed fluid upstream of the wellhead.

A system may include a wellhead and at least one production riserextending into at least one wellbore and configured to convey productionfluid from the wellbore to the wellhead. The system may also include atleast one eductor configured to mix a slip stream of the productionfluid with high-pressure motive fluid, providing a mixed fluid. Thesystem may further include at least one motive fluid line extending intothe wellbore from at least one compressor to the eductor and configuredto provide the high-pressure motive fluid to the eductor. The system mayinclude at least one suction line configured to provide the slip streamfrom the production riser to the eductor. The system may also include atleast one mixed fluid line configured to convey the mixed fluid from theeductor to the production riser.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a prior art cross sectional side view of an apparatusincluding an eductor, in accordance with the present invention.

DETAILED DESCRIPTION

It is proposed that the cause of poor performance of traditional gaslift might be that the gas is poorly distributed as it enters the subseariser. Specifically, it is thought that the gas enters the riser in acontinuous stream forming large gas bubbles which move fast as comparedto the liquid velocity, thus failing to contribute effectively to thereduction in average fluid gradient. It is believed that the use of aneductor downstream of the wellhead may allow for improved gasdistribution through the subsea riser and lower riser base pressure,enhancing the overall rate of production. More specifically, the eductormay generate small gas bubbles in the production stream prior to theproduction stream entering the subsea riser. An eductor may provide awell-mixed dispersion of injection gas and fluid drawn from theproduction riser through a suction line and that dispersion may beintroduced back into the production riser to provide lift.

Referring to FIG. 1, an apparatus may include a production riser 1, aneductor 2, a motive fluid line 3, a suction line 4, and a mixed fluidline 5. Generally, production fluid 6 moves from the pay zone, throughthe production riser 1, through wellhead 7, through a subsea riser 8 andto a floating vessel or other host facilities for further processing,treatment, or transport. The improvements described herein may reduceaverage density, reduce back pressure, and enhance production. Suchimprovements may mitigate flow instabilities which can enhance theprocessing capacity of the host facilities. Additionally, in otherapplications (not shown), an alternate to production fluid 6 may bereturn fluid from drilling or other subsea operations where lighteningof such fluid is desirable to assist movement up an alternate conduit toproduction riser 1.

In the specific example of FIG. 1, production fluid 6 from the wellhead7 is split into a slip stream 9 and a main stream 10. The volume of theslip stream 9 will be controlled by suction created by the eductor 2and/or valve 11. The slip stream 9 may be drawn, via the suction line 4to the eductor 2 for combination with high-pressure motive fluid 12(e.g., high-pressure gas).

The eductor 2 uses the high-pressure motive fluid 12 and the splitstream 9 as an inlet fluid to provide a mixed fluid 13. The eductor 2may be configured to mix the slip stream 9 with the high-pressure motivefluid 12, providing the mixed fluid 13. The eductor 2 may include anozzle 14 and a venturi diffuser 15. The nozzle 14 might provide highvelocity and high mixing. It is important in the mechanical layout toensure that the location of the slip stream 9 is in a place of maximumliquid hold-up with the nozzle 14 aligned with gravity in order tomaximize the liquid portion of slip stream 9.

The high-pressure motive fluid 12 passes through the nozzle 14 of theeductor 2 and then joins the slip stream 9 to create the mixed fluid 13.The mixed fluid 13 passes through the venturi diffuser 15 before flowinginto the mixed fluid line 5, passing by a valve 16 before joining themain stream 10 in the production riser 1. The mixed fluid 13 incombination with the main stream 10 form a lightened fluid 17 that maypass more easily toward a floating vessel or other host facilities.

The production riser 1 may extend into a wellbore (not shown) which maybe cased or uncased. The production riser 1 may be configured to conveyproduction fluid 6 from the wellbore to the wellhead 7 at the sea floor(not shown). The design of the production riser 1 might be substantiallythe same as conventional production risers with the exception of theprovision of ports or openings for fluid communication between theproduction riser 1 and the eductor 2. For example, the slip stream 9from the production fluid 6 might pass into the suction line 4, leavinga main stream 10 to pass upward through the production riser 1.

The slip stream 9 may pass through the suction line 4, optional valve11, and into the eductor 2. Thus, the suction line 4 may be configuredto provide the slip stream 9 from the production riser 1 to the eductor2. A typical system may have lines 4 and 5 being around half of the mainriser size (e.g., an 8″ riser may require 4″ conduits for lines 4 and 5to accommodate the slip stream 9 and mixed fluid stream 13). Velocitiesat the vena contracta at the outlet of the nozzle 14 will approach thespeed of sound and pressure at this point will be below the pressure inthe production riser 1. This reduction in pressure is a function of theacceleration of the motive fluid through the venturi orifice and ismostly recovered in the venturi diffuser 15 with the reduction invelocity and deceleration. In this case the pressure in motive fluidline 3 must be at minimum 2.5 times the pressure of the production fluid6.

The high-pressure motive fluid 12 may be fed to the eductor 2 via themotive fluid line 3. The motive fluid line 3 may be configured toprovide the high-pressure motive fluid 12 to the eductor 2. The motivefluid line 3 may extend into the wellbore from a compressor 18 to theeductor 2. The compressor 18 may be provided outside the wellbore (e.g.,above the surface), with the motive fluid line 3 extending into thewellbore. The high-pressure motive fluid 12 may be produced gas (i.e.,associated gas) or an inert gas. The compressor may be a standard gascompressor found in most oil production facilities. It is alsoconceivable but unlikely that the compressor would be a subseacompressor, but likely not located in a well stream. In some situationsthere may be a high pressure gas riser taking produced gas from separatewells to the production host. This is not all that uncommon, many hostshave multiple fields tied-back, some fields may be higher pressure andhigher gas content and could be used to provide the high-pressure motivefluid such that motive fluid line is connected to a nozzle on the gasriser PipeLine End Termination (PLET) assembly and is routed to themotive fluid line 3 and eductor 2 on the new PLET. Thus, thehigh-pressure motive fluid 12 of the illustrated example may beprovided, at least in part, from a different riser tied in to the motivefluid line 3 subsea. In some instances, the high-pressure motive fluid12 may be a multiphase liquid and gas combination with a gas to oilration substantially higher than the gas to oil ratio of the productionfluid upstream of the motive fluid line 3 and the pressure of thehigh-pressure motive fluid 12 may be at least twice the pressure of theproduction fluid upstream of the motive fluid line 3.

The mixed fluid line 5 may be configured to convey the mixed fluid 13from the eductor 2, optionally through the valve 16, and into theproduction riser 1. Thereafter, the mixed fluid 13 may combine with themain stream 10, creating the lightened fluid 17. The lightened fluid 17may have superior flow properties through a subsea riser 8 that conveysproduction fluid from the wellhead 7 through a body of water to hostfacilities (not shown). Specifically, it is believed that the lightenedfluid 17 has better flow properties as compared with untreatedproduction fluid 6. For example, it is believed that a better gasdispersion may be present in the subsea riser 8, when using the eductor2 upstream of the wellhead 7, as compared to traditional methods.

As illustrated, the mixed fluid line 5 is configured to convey the mixedfluid 13 from the eductor 2 to the production riser 1 at a pointupstream of the suction line 4. In one optional variation of theillustrated example, the mixed fluid 13 may be introduced at a pointupstream of the suction line 4. Alternatively, the mixed fluid 13 may beintroduced at multiple points, in which case those points might all beupstream of the suction line 4, all points might be downstream of thesuction line 4, or some points may be upstream of the suction line 4while other points are downstream of the suction line 4. Similarly,multiple slip streams 9 might be provided in various configurations andpositions relative to the mixed fluid line 5.

As illustrated, the suction line 4 and the mixed fluid line 5 are bothupstream of the wellhead 7. Some or all such lines may connect to theproduction riser inside the wellbore. Alternatively, such connectionsmay be at or near a Christmas tree below the surface of the sea butoutside the wellbore.

In other embodiments which are not illustrated for the sake ofsimplicity, multiple eductors 2 may be provided or the single eductor 2may be used to balance or lighten production fluid from multiplewellbores. Thus, a system may include the wellhead 7, at least oneproduction riser 1 as described above, at least one eductor as describedabove, at least one motive fluid line as described above, at least onesuction line as described above, and at least one mixed fluid line asdescribed above. The placement of the eductor or eductors 2 will bedependent on the specifics of the wellhead, Christmas tree, andproduction riser or risers 1. For example, several eductors 2 may beplaced in series or parallel at or near the Christmas tree, but stillupstream of the subsea riser 8, such that the subsea riser 8 need onlyconvey the lightened fluid 17. Notably, even if only one eductor 2 isused in only one production riser 1, the lightened fluid 17 from thatriser 1 may be combined with untreated production fluid 6 from anotherproduction riser 1 with a beneficial effect. In other words, when thelightened fluid 17 from one wellbore is combined with “regular” fluidfrom another wellbore, the combined fluid in the subsea riser 8 maystill have comparatively better flow properties as compared totraditional methods. Thus, it is not necessary to include a separateeductor 2 for each wellbore or production riser 1.

Various methods of reducing base pressure in the subsea riser 8 are thusapparent. For example, one such method may include providing the eductor2 upstream of the wellhead 7, the wellhead 7 being upstream of thesubsea riser 8. The eductor 2 may be in the wellbore, or might be at alocation above the wellbore (e.g., where streams from multiple wellboreshave been combined). Providing the eductor 2 may include connecting theeductor 2 to the production riser 1 or otherwise placing the eductor 2in a position to draw fluid from a production stream and replace thedrawn fluid with a mixed gas and fluid stream.

The method may further include supplying the eductor 2 withhigh-pressure motive fluid 12. For example, this may involve providingthe motive fluid line 3 with the compressor 18 in communicationtherewith. The high-pressure motive fluid 12 may thus be conveyed fromthe compressor 18 to the eductor 2 via the motive fluid line 3.

The method may include mixing the high-pressure motive fluid 12 andproduction fluid to produce the mixed fluid 13. For example, the eductor2 may include the nozzle 14 and the venturi diffuser 15 to providethorough mixing of the slip stream 9 and the high-pressure motive fluid12.

Finally, the method may include introducing the mixed fluid 13 upstreamof the wellhead 7. For example providing the mixed fluid line 5configured to convey the mixed fluid 13 from the eductor 2 and simplyallowing the mixed fluid 13 to flow from the eductor 2 and join the mainstream 10 to create the lightened fluid 17.

Notably, closing the valve 11 may allow system to still function as aconventional system.

As compared to conventional methods, it is believed that the use of theeductor 2 will provide for better dispersion of high-pressure motivefluid 12 (e.g., high-pressure gas) in fluid (e.g., production fluid 6)such that the corresponding lightened fluid is better infused with gasas compared to similar fluid provided by conventional gas lifttechnology. Thus, more efficient lift may be achieved. This advantagebecomes greater as the riser diameter increases.

1. An apparatus comprising: a production riser extending into a wellboreand configured to convey production fluid from the wellbore to awellhead; an eductor configured to mix a slip stream of the productionfluid with high-pressure motive fluid, providing a mixed fluid; a motivefluid line extending into the wellbore from a compressor to the eductorand configured to provide the high-pressure motive fluid to the eductor;a suction line configured to provide the slip stream from the productionriser to the eductor; and a mixed fluid line configured to convey themixed fluid from the eductor to the production riser.
 2. The apparatusof claim 1, further comprising a valve in the suction line.
 3. Theapparatus of claim 1, further comprising a valve in the mixed fluidline.
 4. The apparatus of claim 1, wherein the mixed fluid line isconfigured to convey the mixed fluid from the eductor to the productionriser at a point downstream of the suction line.
 5. The apparatus ofclaim 1, wherein the suction line and the mixed fluid line are eachdownstream of the wellhead.
 6. The apparatus of claim 1, wherein thecompressor is outside the wellbore.
 7. The apparatus of claim 1, whereinthe high-pressure motive fluid is provided, at least in part, from adifferent riser tied in to motive fluid line subsea.
 8. The apparatus ofclaim 7, wherein the motive fluid is a multiphase liquid and gascombination with a gas to oil ratio substantially higher than the gas tooil ratio of the production fluid upstream of the motive fluid line; andwherein the pressure of the high-pressure motive fluid is at least twicethe pressure of the production fluid upstream of the motive fluid.
 9. Amethod for reducing base pressure in a subsea riser, comprising:providing an eductor upstream of a wellhead, wherein the wellhead isupstream of the subsea riser; supplying the eductor with high-pressuremotive fluid; supplying the eductor with production fluid; with theeductor, mixing the high-pressure motive fluid and the production fluidto produce a mixed fluid; and introducing the mixed fluid upstream ofthe wellhead.
 10. The method of claim 9, wherein providing the eductorcomprises connecting the eductor to a production riser.